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Evaluating Wireless LAN Access Methods in Presence of Transmission - - PowerPoint PPT Presentation
Evaluating Wireless LAN Access Methods in Presence of Transmission - - PowerPoint PPT Presentation
Evaluating Wireless LAN Access Methods in Presence of Transmission Errors IEEE INFOCOM 2006, Poster session Elena Lopez-Aguilera Martin Heusse Franck Rousseau Andrzej Duda Jordi Casademont LSR-IMAG Outline Introduction Principles
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Introduction
1997: IEEE defines the first standard IEEE 802.11 for Wireless Local Area Networks
Successive variants have increased the nominal bit rate: IEEE 802.11 b/g/a The MAC layer remains unchanged Much research effort spent on improving MAC performance
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Introduction
IEEE 802.11 Distributed Coordination Function
Before initiating a transmission, a station senses the channel during a DIFS Time:
the medium is sensed idle → transmission allowed the medium is sensed busy → next attempt of transmission at DIFS + backoff time
Backoff time: integer number of time slots distributed uniformly in [0, CW-1] After each data frame succesfully received, the receiver transmits an ACK after a SIFS Time
Data DIFS Tx ACK SIFS Data DIFS + backoff Tx ACK SIFS Medium idle Medium busy
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Chosen Access Methods
Different MAC proposals for improving IEEE 802.11 Wireless LANs
Slow Decrease Asymptotically Optimal Backoff (AOB) Idle Sense
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Principles of chosen Access Methods
Slow Decrease
Objective: adapting CW of each station to the current network congestion level After each successful transmission:
the slowest decrease, which achieves the best performance, for
g=1 → Preserves the exponential backoff mechanism of IEEE 802.11 DCF
) 2 , max(
min
- ld
g new
CW CW CW
−
=
- ld
new
CW CW ⋅ = 5 .
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Principles of chosen Access Methods
Asymptotically Optimal Backoff (AOB)
Each host computes the Probability of Transmission: Na: Number of attempts for the transmission of a frame Slot Utilization (SU): If the transmission is rescheduled, a new backoff interval is computed
AOB preserves the exponential backoff mechanism of IEEE 802.11 DCF
Na
- pt
SU SU PT − = , 1 min 1
Slots Available Num Slots Busy Num SU _ _ _ _ =
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Principles of chosen Access Methods
Idle Sense
Each host estimates the number of consecutive idle slots between 2 transmission attempts By comparing the estimate with a target value, hosts adjust their CW using AIMD principle Contending hosts do not perform the exponential backoff mechanism of IEEE 802.11 DCF
Up to now, the different proposals have been compared under ideal channel conditions
Objective: Performance analysis of the different proposals in adverse transmission conditions
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Simulation environment
Simulation parameters
Physical layer of IEEE 802.11g 1 BSS: every station subject to the same BER FER=1-(1-BER)l FER: Frame error ratio; l: frame size in bits Payload size of 1500 bytes and transmission rate of 54 Mbps Greedy hosts
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- Aggregate Throughput vs. number of stations
BER=10-5, FERData=12%, FERACK=0.65% Throughput gain with
Idle Sense (%): 3.9 % for 10 stations 35.6 % for 100 stations
System performance
18 20 22 24 26 28 30 20 40 60 80 100 Aggregate Throughput (Mbps) Number of stations IEEE 802.11 DCF Idle Sense Slow decrease AOB
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- Number of idle slots vs. number of stations
BER=10-5, FERData=12%, FERACK=0.65%
System performance
2 4 6 8 10 20 40 60 80 100 Number of idle slots Number of stations IEEE 802.11 DCF Idle Sense Slow decrease AOB Target
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- Channel Access Fairness: Jain Index
Number of stations = 25, BER=10-5, FERData=12%, FERACK=0.65%
System performance
0.2 0.4 0.6 0.8 1 10 20 30 40 50 Jain index Normalized window size IEEE 802.11 DCF Idle Sense Slow decrease AOB
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System performance
AOB and Idle Sense provide significant improvement of the throughput performance Idle Sense
number of idle slots closer to the target than AOB better Channel Access Fairness
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- Aggregate Throughput vs. number of stations
BER=10-4, FERData=72%, FERACK=6.4% Throughput gain with
Idle Sense (%): 127 % for 2 stations 60.3 % for 4 stations 15.4 % for 10 stations 3.6 % for 20 stations
System performance
2 4 6 8 10 20 40 60 80 100 Aggregate Throughput (Mbps) Number of stations IEEE 802.11 DCF Idle Sense Slow decrease AOB
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System performance
- Number of idle slots vs. number of stations
BER=10-4, FERData=72%, FERACK=6.4%
10 20 30 40 50 60 20 40 60 80 100 Number of idle slots Number of stations IEEE 802.11 DCF Idle Sense Slow decrease AOB Target
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- Fairness: Jain Index
Number of stations = 25, BER=10-4, FERData=72%, FERACK=6.4%
System performance
0.2 0.4 0.6 0.8 1 10 20 30 40 50 Jain index Normalized window size IEEE 802.11 DCF Idle Sense Slow decrease AOB
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System performance
Idle Sense
the best overall throughput performance number of idle slots closer to the target: it does not perform the exponential backoff algorithm better Channel Access Fairness
Slow Decrease and AOB:
do not improve the IEEE 802.11 DCF performance perform the exponential backoff after collisions and frames losses
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